The present invention relates to interference control techniques in Code Division Multiple Access (xe2x80x9cCDMAxe2x80x9d) communication systems.
CDMA systems transmit a plurality of different information signals to receivers in xe2x80x9clogical channels.xe2x80x9d The logical channels typically are transmitted simultaneously on a single radio channel. Each logical channel includes an information signal that has been modulated by an orthogonal (or quasi-orthogonal) code, called the xe2x80x9cchannel code.xe2x80x9d The logical channels are summed to build an aggregate signal for transmission. When received, a CDMA receiver obtains the information signal from the logical channel by synchronously correlating the received aggregate signal with the channel code. Due to the correlation function, all other logical channels are reduced to low level noise and the channel code for the one logical channel cancels out. The information signal from the logical channel thus is retrieved. (CDMA systems are those referred to as xe2x80x9cinterference limitedxe2x80x9d systems since the signal process of the other logical channels may not completely cancel.)
CDMA systems may be deployed in spatial regions of varying scales. It is known to provide wireless CDMA local area networks (xe2x80x9cLANsxe2x80x9d) within an office building. CDMA cellular systems, such as the known xe2x80x9cIS-95xe2x80x9d system, may be deployed on a city-wide or even nationwide basis. And CDMA satellite systems may provide global coverage.
In a cellular system, a spatial area may be divided into a plurality of cells, each with its own base station transmitter/receiver (xe2x80x9cbase stationxe2x80x9d). The base station communicates with one or more mobile stations (xe2x80x9cmobilesxe2x80x9d or xe2x80x9csubscribersxe2x80x9d). To distinguish the channels of one base station from the channels of another, the aggregate sum of logical channels for one base station are modulated by another code before transmission, called the xe2x80x9cbase station code.xe2x80x9d Among the base stations, the different base station codes are orthogonal or quasi-orthogonal. A cellular CDMA receiver may perform two correlation functions: A first correlation function isolates signals from a first base station from other base station signals that may have been received, the signals from the other base stations are reduced to low level noise. A second correlation function isolates signals from a first logical channel transmitted by the base station from other logical channels transmitted by that base station, as the signals from the other logical channels are reduced to low level noise. Finally, dispersive effects contribute to additional noise.
The capacity of a CDMA system is said to be xe2x80x9cinterference limitedxe2x80x9d because the low level noise generated by the other logical channels may disturb the quality of the information signal obtained from the one logical channel to which the receiver is xe2x80x9ctuned.xe2x80x9d The amount of interference obtained from the other signals is directly related to the received power of those signals. Thus, many CDMA systems employ sophisticated power control techniques to limit unnecessary transmitted power of CDMA signal and avoid xe2x80x9ccross-talk,xe2x80x9d the interference that a first logical channel assert on other logical channels.
Of course, a first logical channel must be transmitted with sufficient energy so that the information signal within may be retrieved therefrom. Thus, regardless of the interference that it may cause to other logical channels, a transmission from a base station to a distant subscriber, such as one that is on the edge between two cells, must be transmitted with sufficient power to convey intelligible information to the distant subscriber.
There is a need in the art for an improved interference mitigation scheme in a CDMA communication system that permits high power transmissions to be made for distant subscribers but also minimizes the total transmitted power in the system.
Embodiments of the present invention provide an interference mitigation method for a CDMA system in which subscribers that will receive a high power transmission generate an alert message to surrounding base stations. A surrounding base station, before it generates new transmissions, determines the distance between it and a second subscriber that will receive the new transmission. If the distance is greater than an interference threshold, the base station delays the new transmission until the first high-power transmission concludes. The method prevents two high-power transmissions from occurring simultaneously.